In daily life, for exactly identifying environment and directions in the dark, illumination devices have already been sorts of indispensable tools. Due to that the cost of energy sources usually keeps in high, and the power used in illumination in a certain proportion of overall power in global for a long time, there are many engineers continuously doing their efforts to research and develop proper illumination devices to reduce the power consumption cost for illumination.
Additionally, with respect to the function of the illumination device, the most illumination devices still needs to connect to an external power, such as public power supplying system, and then provides illumination to an external environment. However, once the external power source interrupts supplying a specified power to the illumination device, more particular when a large electric power cut occurs after a natural disaster, the most illumination device cannot go on providing emergent illumination to the external environment.
For saving the cost of power used for illumination and providing the function of emergent illumination, there are many light devices being designed and widely used for most of people. Following up, two representative prior embodiments are cited to do more detail description.
Please refer to FIG. 1, which is a functional block diagram illustrating a conventional light device being capable of being charged by a solar energy system. As shown in FIG. 1, a light device comprises a chargeable power source 11, a circuit interface 12 and a lighting element 13. The circuit interface 12 is respectively connected to the chargeable power source 11 and the lighting element 13. The chargeable power source 11 is applied to discharge a specified power to the lighting element 13, so as to make the lighting element 13 project an illumination light beam.
When the power of the chargeable power source 11 is insufficient to drive the lighting element 13 to project an illumination light beam to the external environment, it is able to electrically connect the chargeable power source 11 to the solar energy system 2, so as to charge the chargeable power source 11. After completing the charging operation to the chargeable power source 11, the chargeable power source 11 can discharge the specified power to the lighting element 13, which can go on projecting the illumination light beam to the external environment.
People skilled in related arts can easily realize that, comparing with the petrochemistry or nuclear energy power generation means, the solar energy can be gained everywhere to save the energy cost. However, the supplying of solar energy is not very stable, so that it is unable to ensure that the power stored in the solar energy system can stably supply to the chargeable power source 11, and it is further unable to ensure that the lighting element 13 can provide stable illumination to the external environment.
Please refer to FIG. 2, which is a functional block diagram illustrating that another light device can provide emergent illumination. As shown in FIG. 2, a light device 1a comprises a circuit interface 12a, the chargeable power source 11 and the lighting element 13. The circuit interface 12a comprises a power supplying circuit 121a, a switch circuit 122a, a power distribution circuit 123a and a detecting circuit 124a. The power supplying circuit 121a is electrically connected with the power distribution circuit 123a and the lighting element 13, and the switch circuit 122a is electrically connected to the chargeable power source 11 and the detecting circuit 124a. 
When the light device 1a is connected to an external power source 3, the power distribution circuit 124a can detect whether the external power source 3 sends a specified power to the power distribution circuit 123a. If the detecting circuit detects that the external power source 3 sends the specified power to the power distribution circuit 123a, the detecting circuit 124a can send a normal switch signal S1 to the switch circuit 122a, so that the chargeable power source 11 can electrically connect to the power distributed circuit 123a via the switch circuit 122a. 
At this moment, the specified power supplied from the external power source 3 can be sent to the power distribution circuit 123a, where can divide the specified power into a first distributed power and a second distributed power according to a preset distribution proportion. The first distributed power is sent to the lighting element 13 via the power supplying circuit 121a, so as to drive the lighting element 13 to project the illumination light beam. The second distributed power is sent to the chargeable power source 11 to charge the chargeable power source 11, where stored with an emergent power.
On the contrary, if the detecting circuit 124a detects the external power source 3 interrupting sending the specified power to the power distribution circuit 123a, the detecting circuit 124a can send an emergent switch signal S2 to the switch circuit 122a to connect the chargeable power source 11 with the power supplying circuit 121a via the switch circuit 122a. At this moment, the emergent power stored within the chargeable power source 11 can be sent to the lighting element 13 via the switch circuit 122a and the power supplying circuit 121a, so as to drive the lighting element 13 to project the illumination light beam.
Except for of the functional block diagram as shown in FIG. 2, in some circuit designs, the detecting circuit 124a is further electrically connected to the chargeable power source, so as to detect whether the emergent power stored within the chargeable power source reaches to a nominal power. When the emergent power reaches to the nominal power, the detecting circuit 124a can send a distribution signal to the switch circuit and the power distribution circuit 123a, so as to interrupt the chargeable power source 11 being electrically connected to the power distribution circuit 123a, and adjust the preset distribution proportion.
People skilled in related arts can easily realize that, in the above technology, a circuit design for a light device is disclosed, wherein when the external power source 3 interrupts sending the specified power, it is able to drive the lighting element 13 to provide emergent illumination via the emergent power supplied from the chargeable power source 11.
However, in the above technology, for making the chargeable power source be stored with sufficient emergent power source, the power distribution circuit has to divide the specified power into the first distributed power and the second distributed power according to the preset distribution proportion, and then sends the second distributed power to the chargeable power source 11. Thus, it must reduce the utilization ratio of the power used to light, wherein the utilization ratio of the power is related to the value that divides the first distributed power by the specified power.